1. Field of the Invention
The present invention relates to a flat-panel photosensor that includes an active matrix type TFT array substrate, on which a photodiode for converting visible light into electric charges and a thin-film transistor (hereinafter referred to as a TFT) employed as a switching element are arranged like a matrix.
2. Description of the Related Art
A flat-panel photosensor that includes a TFT array substrate, on which a photodiode for performing photoelectric conversion of visible light and a TFT are arranged, is applied as a contact image sensor or an X-ray image display apparatus, and is widely employed. Especially, a flat-panel X-ray image display apparatus (hereinafter referred to as an FPD), which is constituted by providing, on a TFT array substrate, a scintillator that converts X rays into visible light, is a favorable apparatus for the application to, for example, a medical industry.
In the X-ray image diagnosis field, observation of a detailed image (still image) and observation of a real-time image (moving picture) are performed for different purposes. An X-ray film is still employed mainly to obtain a still picture, while an image pickup tube (image intensifier) that includes a photomultiplier and a CCD is employed to obtain a moving picture. The X-ray film provides a high spatial resolution, but there are several drawbacks: the X-ray film has a low sensitivity and enables radiographing of only still pictures, and since the developing process is required after radiographing, this is not appropriate for instantaneous use. On the other hand, the image pickup tube provides a high sensitivity and enables radiographing of a moving picture. However, the image pickup tube has a low spatial resolution, and since this is a vacuum device, the increase of the device size is limited.
For an FPD, there are an indirect conversion type that converts X rays into light using a scintillator, such as CsI, and then converts the light into electric charges using a photodiode, and a direct conversion type that directly converts X rays into electric charges using an X-ray detection element, such as Se. The indirect conversion type provides high quantum efficiency and a superior signal/noise ratio, and requires only a small amount of exposure to perform X-ray radiographing and image pickup. The structure of the array substrate of the indirect conversion type FPD and the manufacturing method thereof have been disclosed (see, for example, JP-A-2004-63660 (FIG. 9), JP-A-2004-48000 (FIG. 4) and JP-A-2003-158253 (FIG. 1)).
Formation of a photodiode that influences the sensitivity of a photosensor and noise is important for the array substrate of an FPD. A photosensor includes amorphous silicon layers deposited on an electrode. For example, as in JP-A-2004-63660, when a photosensor is formed on a cathode electrode that includes the same layer as the gate electrode of a thin film transistor, the following problem has risen. Specifically, when a lower electrode 609 of a photodiode is formed of the same material as used for a gate electrode layer, the electrode 609 receives more damage caused by dry etching, or caused by forming a source electrode layer 605 and a drain electrode layer 606, because the electrode 609 is located at the lowermost as well as the gate electrode layer. Accordingly, surface roughness occurs and a leak current from the photodiode is increased. In order to avoid this problem, for example, high-melting-point metal must be employed to form the lower electrode 609 of the photodiode. In this case, an aluminum alloy film having a low resistance can not be employed to form a gate electrode and gate wiring. Further, a margin of an opening size will be reduced for connection between the source electrode layer 605 and the cathode electrode layer 609. In order to avoid this problem, as disclosed in JP-A-2004-48000 or JP-A-2003-158253, the lower electrode for a photosensor might be formed on an electrode that is formed of the same layer as the source electrode and the drain electrode of a thin film transistor.
One of the methods for improving the output performance of the photosensor is a method for increasing the ratio of the dimension of an Si layer, which serves as an photodiode, to the dimension of one sensor component. Therefore, in a conventional structure described in, for example, JP-A-2004-48000 or JP-A-2003-158253, a photodiode encloses a contact hole that is a diode bottom contact opening. That is, the Si layer that constitutes a photodiode is formed to cross the edge of a contact hole. However, in this structure, it is found through our evaluation that, when the opening edge length obtained by adding the lengths of edges, i.e., the circumferential length of the opening of the contact hole was increased, the current leak element was increased. As this reason, we consider that, when a step difference formed by a contact hole is present in a formation area for an Si layer that constitutes a photodiode, the Si layer unevenly grows at the step difference, or a membrane stress is generated in the Si layer at the step difference. Since the increase of a leak current deteriorates the sensitivity of a photosensor, inhibition of a leak current is indispensable.